The present invention relates to the liquefaction of coal to produce a normally liquid product which is low in sulfur and nitrogen and has a particularly high API gravity.
As a consequence of the increasing costs and diminishing supplies of petroleum much research is being conducted into better ways of obtaining synthetic fuels from solids such as coal. Furthermore, as a consequence of increased emphasis on the reduction of air pollution, fuels with low sulfur and low nitrogen contents are in great demand. Unfortunately, however, most coals contain large amounts of sulfur and nitrogen which end up in the synthetic liquids produced from the coal which necessitates additional costly sulfur and nitrogen removal steps, further increasing the costs of the synthetic fuels.
Numerous processes are well known in the art for the production of liquid products from coal.
In many processes for coal liquefaction, hydrogen is supplied by a liquid donor solvent. In such processes, the function of any catalyst is to rehydrogenate the solvent by adding molecular hydrogen to it. Thus the solvent acts as a medium to carry hydrogen from the catalyst to the solid coal. However, in such processes the catalyst is typically rapidly deactivated with the result that the process is highly inefficient and not conducive to a commercial coal hydrogenation process.
Another problem with prior art processes results from the insoluble solids which are contained in the liquid product. Typically, the liquid product from a coal liquefaction process has a high molecular weight. The high molecular weight of the product makes it very difficult to separate the very fine insoluble solids (coal residue). Furthermore, it has generally been taught that these insoluble solids must be separated prior to further processing in order to prevent downstream catalyst deactivation.
A further problem of prior art coal liquefaction processes is that the normally liquid product typically contains 0.2 to 1.0 or more weight percent sulfur and nitrogen. These potential pollutants must be removed in order to produce a valuable clean fuel and the removal of these contaminants requires costly additional hydroprocessing steps which further increase the cost of the product.
Typical of the prior art processes is the Gulf catalytic coal liquefaction process, disclosed in Coal Conversion Technology, Smith et al, Noyes Data Corporation (1976), where a slurry of coal and a process-derived solvent is forced up through a bed of catalyst at 900.degree. F. and 2000 psig. The product, as taught in Sun W. Chun, National Science Foundation, Ohio State University Workshop, "Materials Problems and Research", Apr. 16, 1974, has a gravity of 1.2.degree. API, a sulfur content of 0.11 weight percent, and a nitrogen content of 0.63 weight percent.
Another typical and well-known prior art process is the Synthoil process wherein a coal solvent slurry is pumped into a catalytic fixed bed reactor with hydrogen at a high velocity. Similar to the Gulf process, the Synthoil process also produces a liquid product, as taught in "Coal Liquefaction", Sam Friedman et al, presented at NPRA National Fuels & Lubricants Meeting, Nov. 6-8, 1974, Houston, Texas, which has a gravity of -0.72.degree. API and a sulfur content of 0.2 weight percent.